Created attachment 8600
It was noticed that on login with LightDM there is significant screen tearing. The first login has "common" (hard to find an accurate word) graphical bugs from this GPU when bugs happen, having a mostly gray with red and green erratic texture. After the first login, subsequent logins show different patterns for screen tearing, based mainly on previous images being shown on screen, for example, there is https://i.imgur.com/DgYIXcc.jpg which was pretty mild, showing only the desktop corrupted. This bug could allow one user to have an idea of what the previous user was seeing: I managed to read the pidgin channel names I was on in the previous login once.
Downgrading xfwm4 to the Xubuntu 19.04 default makes the bug disappear. No screen tearing was observed in other moments, it was only on the login screen. I had never observed screen tearing on this computer before.
$ xfwm4 --version
This is xfwm4 version 4.13.2 (revision f20655b8) for Xfce 4.12
Released under the terms of the GNU General Public License.
Compiled against GTK+-3.24.8, using GTK+-3.24.8.
Build configuration and supported features:
- Startup notification support: Yes
- XSync support: Yes
- Render support: Yes
- Xrandr support: Yes
- Xpresent support: Yes
- Embedded compositor: Yes
- Epoxy support: Yes
- KDE systray proxy (deprecated): No
OS: Xubuntu 19.04 using Xubuntu-dev PPA
Xfce version: 4.14pre1
CPU: Intel Core i3-540
GPU: AMD Radeon HD 5450
RAM: 4 GB
Monitor connections affected: VGA
If you get tearing with GL, I'm afraid there nothing I can do.
xfwm4 uses `glXSwapBuffers()` which swaps buffers during the vertical retrace of the monitor. If you get tearing, it's a driver issue.
You could try with XPresent to see if that's better, from a terminal, try:
$ xfwm4 --replace --vblank=xpresent
If that solve the tearing issue, you can make “xpresent” the default vblank method with:
$ xfconf-query -c xfwm4 -p /general/vblank_mode -s xpresent
Changing the vblank_mode to xpresent or off solves the screen tearing problem. The screen tearing on login happens when using auto or glx options.
Basically “auto” == “glx” so it glx doesn't work, neither will auto.
Closing, as Xpresent works.